Laser configuration



1968 c. J. KOESTER 3,369,192

LASER CONFIGURATION Filed July 27, 1962 Fig. l.

ANGLE 6 SURF/ICE [/5 /7 "/5 CONT Q01 T E POWER SOURCE INVENTOR. Ci /xmfsff (055756 ATTORNEY United States Patent 3,369,192 LASER CONFIGURATIONCharles J. Koester, South Woodstock, Conn., assignor, by mesneassignments, to American Optical Company, Southbridge, Mass., acorporation of Delaware Filed July 27, 1962, Ser. No. 212,818 2 Claims.(Cl. 331-945) This invention pertains to an improved laser configurationand has reference to the general arrangement in which two sphericalreflectors of equal curvature are separated by their common radius ofcurvature, as discussed in Bell System Technical Journal of March 196-1,beginning on page 489.

If a standard Fabry-Perot interferometer is adjusted to satisfy theinterference equation n \=-2t cos 0 at normal incidence 0:0, for aparticular wavelength )0, then for an angle 0 slightly different from 0,the interference condition is satisfied for another wavelength A. In alaser this would lead to a stimulated emission whose wavelength dependedon the angle 0.

With the spherical Fabry-Perot interferometer, howevery, the resonantwavelength would be independent of angle which therefore makes possiblea narrower output line for the laser.

Referring to the drawings:

FIG. 1 is a diagram showing the fundamental arrangement of the sphericalFabry-Perot interferometer which is applied to a laser.

FIG. 2 is a diagrammatic view partially in cross-section of a solidcylindrical laser formed with convex end surfaces and embodying thisinvention.

FIG. 3 is a diagramshowing the waves formed at several radial distancesfrom the center of curvature of the spherical wave. a

FIG. 4 is a diagrammatic view of a conical-shaped laser arrangement alsoembodying this invention.

Special reference may first be had to FIG. 1, in which spaced surfaces 1and 2 form arcs of circles. The center of curvature of surface 2 lies atthe vertex C on surface 1 and the center of curvature of surface 1 liesat the vertex D on surface 2. All rays passing through the point C andstriking surface 2 are reflected directly back on themselves and executethe path indicated by lines with arrows thereon. The interferencecondition is represented by nx=4t,u, where n is an integer, A=wavelengthand t: separation of surfaces along the axis, which is equal to theradius of surface 2, and ,u=index of refraction of the laser material.

It should be noted particularly that the interference equation isindependent of the angle 0 which the ray 3 makes with respect to theaxis 4 of the system (shown in broken lines).

The spherical Fabry-Perot interferometer may have both surfaces 1 and 2coated for high reflection over their whole areas. However, inaccordance with this invention, the arrangement shown in FIG. 2. has adefinite advantage because the number of modes which can be excited isreduced and thereby more of the available pumping energy is put into afew desired modes.

Referring specifically to FIG. 2:

A solid laser material such as a ruby is formed into a cylinder 5 andhas its end surfaces 6 and 7 curved as are surfaces 1 and 2 in FIG. 1,with the center of curvature of each at the point where the axis 8intersects the other.

The surface 7 is provided with a silver coating 10 or is otherwise madea high reflectance area.

As here shown, however, the surface 6 has only a small high reflectancearea at 11 and the remainder of this surface is preferably roughened orabsorbing as is the outer cylindrical wall of the cylinder 5.

3,369,192 Patented Feb. 13., 1968 With this arrangement the only wavewhich is reinforced by inter-reflections is essentially a portion of aspherical wave with its center at 11. Other waves are lost eitherthrough the sides or through the low reflection area of surface 6.

For best mode selection the sides of the cylinder should be roughened toprevent them from being totally internally reflecting.

The laser cylinder 5 is surrounded by a helical flash tube 15 whichfurnishes the pumping energy to actuate the laser. The tube 15 isconnected to a suitable source of electric power 16, marked PowerSource, and the usual trigger circuit 17 is looped through the turns ofthe flash tube and joined to the source of trigger voltage 18, markedControl.

The laser output would be an intense spherical wave with its center at11, and if this is assumed to be obtained from the right end 10 in FIG.2, it may be collimated by lens 19.

In addition to the advantage of mode selection which I is attained bythe use of this invention, the pumping light may, if desired in aparticular application, be brought in with very little loss through thelow-reflection area of surface 6, as well as through the side walls aspreviously mentioned.

FIG. 3 shows diagrammatically several wave fronts 20, 21 and 22 spacedbetween the surface 7 and the surface 6 to indicate by arrows 23 theinterrefiections of waves which reinforce the laser output shown in FIG.2.

FIG. 4 illustrates a modification which also embodies this invention,but instead of a solid laser in the form of a cylinder with convex ends,a solid laser 25 has a frustoconical shape with the larger end having aconvex surface 26 silvered at 27 or otherwise reflective, and thesmaller end having a concave surface 28, preferably silvered, with acurvature concentric with the convex surface 26.

The center of curvature 30 is at a radial distance T' from surface 28and 1- from surface 26. This arrangement has the advantage that the raysare not concentrated at a small area reflector, as 11 in FIG. 2, and anytendency to overheat is reduced.

One laser material, artificial ruby, is birefringent. If made in theform of a spherical Fabry-Perot interferometer, as shown in FIG. 2, thenin general there would be two wavefronts with origin at 11. One, theordinary wavefront, would be spherical. The other, the extraordinarywavefront, would be an ellipsoid of revolution. Only the sphericalwavefront will be reinforced by interreflections.

This factmakes it possible to produce substantially all of the laserlight with one state of polarization. Assuming that the optic axis isvertical, then, as shown in FIG. 3, the ordinary wavefront is polarizedhorizontally and the laser output will be an intense spherical wave withits center at 11 horizontally polarized.

If it is desired to produce laser action with the extraordinary wave,the surface 7 may be made nonspherical instead of spherical so as tocause this wave to be reflected back to 11. The proper shape would bethat of the Well known E-wave surface.

If the optic axis is parallel to the axis of the interferometer, theordinary wave will be tangentially polarized The polarized laser outputoffers additional opportunities for modulation. The Faraday or Kerreffect may be used to change the azimuth of polarization or the state ofpolarization from plane to elliptical.

While the illustrated embodiments of FIGS. 2 and 4 include a solid statelaser, a liquid or gaseous state laser may be employed in containerswhich would take the shapes illustrated in FIGS. 2 and 4. Reference hasbeen made to the use of a synthetic ruby as the solid state laser Asolid laser body shaped as herein describedis for most purposesequivalent to a liquid or gaseous body held to the same shape by asuitable container, and in claiming a laser body the broaderconstruction is to be understood except where the claims arespecifically limited to a solid state laser body.

Various modifications of the invention will be apparent to those skilledin this art from the foregoing embodiments, and only such limitationsshould be imposed as are indicated in the appended claims.

I claim:

1. A body formed of laser material, said body having side walls disposedin concentric relation to a longitudinal axis extending therethrough, apair of convexly curved spherical surfaces defining opposite ends ofsaid body, said convexly curved end surfaces being arranged in facingrelation to each other and in predetermined spaced relation along theaxis of said body, the center of curvature of each convexly curved endsurface being located at the intersection of said axis with the other ofsaid convexly curved end surfaces, one of said convexly curved endsurfaces being of relatively high reflectivity over all parts thereofand the other end surface being of relatively low reflectivity over allparts thereof except for a relatively small area immediately surroundingand disposed in concentric relation wtih said axis, and said relativelysmall surface area having a relatively high reflectivity, at least oneof said end surface areas of high reflectivity being slightlytransmissive to optical energy at the emission wavelength of said lasermaterial, whereby an optical resonant cavity for lower order modepropagation only and at increased efficiency will be formed between saidareas of relatively high reflectivity.

2. A cylindrically shaped body of laser material, a pair of convexlycurved spherical surfaces defining the opposite ends of saidcylindrically shaped body, said convexly curved end surfaces beingarranged in facing relation to each other and in predetermined spacedrelation along the axis of said cylindrical body, the center ofcurvature of each convexly curved end surface being located at theintersection of said axis with the other of said convexly curved endsurfaces, one of said convexly curved end surfaces being of relativelyhigh reflectivity over all parts thereof and the other end surface beingof relatively low reflectivity over all parts thereof except for arelatively small area immediately surrounding and disposed in concentricrelation with said axis, and said relatively small surface area having arelatively high reflectivity, at least one of said end surface areas ofhigh reflectivity being slightly transmissive to optical energy at theemission wavelength of said laser material, whereby an optical resonantcavity for lower order mode propagation only and at increased efficiencywill be formed between said end surface areas of relatively highreflectivity.

References Cited UNITED STATES PATENTS 3,055,257 9/1962 Boyd et a1. 8813,114,268 12/1963 Boldridge 33194.5 3,136,959 6/1964 Culver 33194.5

OTHER REFERENCES Gordon et al.: Confocal Multimode Resonator forMillimeter Through Optical Wavelength Masers, Bell System TechnicalJournal, vol. 40, No. 2, March 1961, pp. 489-508.

JEWELL H. PEDERSEN Primary Examiner.

R. L. WILBERT, Assistant Examiner.

1. A BODY FORMED OF LASER MATERIAL, SAID BODY HAVING SIDE WALLS DISPOSEDIN CONCENTRIC RELATION TO A LONGITUDINAL AXIS EXTENDING THERETHROUGH, APAIR OF CONVEXLY CURVED SPHERICAL SURFACES DEFINING OPPOSITE ENDS OFSAID BODY, SAID CONVEXLY CURVED END SURFACES BEING ARRANGED IN FACINGRELATION ON EACH OTHER AND IN PREDETERMINED SPACED RELATION ALONG THEAXIS OF SAID BODY, THE CENTER OF CURVATURE OF EACH CONVEXLY CURVED ENDSURFACE BEING LOCATED AT THE INTERSECTION OF SAID AXIS WITH THE OTHER OFSAID CONVEXLY CURVED END SURFACES, ONE OF SAID CONVEXLY CURVED ENDSURFACES BEING OF RELATIVELY HIGH REFLECTIVITY OVER ALL PARTS THEREOFAND THE OTHER END SURFACE BEING OF RELATIVELY LOW REFLECTIVITY OVER ALLPARTS THEREOF EXCEPT FOR A RELATIVELY SMALL AREA IMMEDIATELY SURROUNDINGAND DISPOSED IN CONCENTRIC RELATION WITH SAID AXIS, AND SAID RELATIVELYSMALL SURFACE AREA HAVING A RELATIVELY HIGH REFLECTIVITY, AT LEAST ONEOF SAID END SURFACE AREAS OF HIGH REFLECTIVITY BEING SLIGHTLYTRANSMISSIVE TO OPTICAL ENERGY AT THE EMISSION WAVELENGTH OF SAID LASERMATERIAL, WHEREBY AN OPTICAL RESONANT CAVITY FOR LOWER ORDER MODEPROPAGATION ONLY AND AT INCREASED EFFICIENCY WILL BE FORMED BETWEEN SAIDAREAS OF RELATIVELY HIGH REFLECTIVITY.